Weakly disordered quantum systems are known to exhibit a universal
behavior of their energy spectrum which depends only on the
symmetry of the system [1]}.
The statistical properties of the energy
spectrum do no depend on the details of the system, and are described by
a random matrix model with the same symmetry. This behavior is extremely
useful in identifying and understanding various properties of metallic
systems [2]}, [3]}, [4]}, [5]}, [6]}.
